1. Field of the Invention
The present invention relates to the recovery of oil from underground deposits thereof, in which the permeability is highly nonuniform, resulting in inefficient sweep when such deposit is subjected to steamflooding or other high temperature operation. More particularly, it is concerned with improving the sweep efficiency in a steamflooding operation by selectively plugging the loose or highly permeable streaks in the matrix with long set-time lignosulfonate gels.
2. Description of the Prior Art
Methods for plugging or reducing the rate of fluid movement in porous media make up a substantial amount of technology, including the in-place formation of gels. One current technique involves selective plugging using time-set alkaline gels of sodium silicate. However, the applicable time-set range of these gels limits the treatment volumes. This limitation of silica solutions is particularly severe at temperatures above 170.degree. F because of the pronounced shortening of the gel times. Also, the presence of formation brines or contacts with an acid medium, such as CO.sub.2, causes the silicate solution to set prematurely. Moreover, the cost of silicate solutions is such that their use in volumes that would really be effective, e.g., at least 5,000 barrels, is limited.
In a commonly assigned U.S. Pat. No. 3,897,827 by Felber et al., a method for selective plugging in porous media utilizing lignosulfonate gels activated by a mixture of dichromate and salt is disclosed. The mixed accelerator lignosulfonate gel system possesses the specific advantage, in addition to being economically feasible, of having sufficiently long set-times prior to gelation to allow use at temperatures characteristic of waterflooding and the like. However, these same compositions of U.S. Pat. No. 3,897,827 have a pragmatic upper limit of about 250.degree. F above which the gel time decreases sufficiently to prohibit large volume oil field treatments. Because of this limitation the mixed accelerator method cannot be used in ultra-hot reservoirs, steamflooding operations or underground combustion techniques.